1. Field of the Invention
The present invention is generally related to a method for determining the amount of resources to be allocated to a forward link secondary channel over the air interface.
2. Description of the Related Art
Communication systems, and in particular, wireless communication systems comprise a plurality of communication links through which subscribers of such systems communicate with each other and with subscribers of other communication systems. Wireless communication systems such as Code Division Multiple Access (CDMA) systems and other communication systems have an air interface through which communication signals between a subscriber and the system are exchanged (i.e., transmitted and/or received). A subscriber exchanges communication signals over the air interface with a base station that serves a particular geographic region known as a cell. A base station generally comprises radio equipment and processing equipment to allow communication with subscribers located within the cell. Typically, wireless communication systems comprise a plurality of cells, each of which has at least one base station that serves subscribers located within the cell. The air interface for each subscriber comprises two communication channels: a forward channel and a reverse channel. In the forward channel, signals from the base station are transmitted to a subscriber. Conversely, a subscriber uses the reverse channel to transmit signals to the base station of the cell within which such subscriber is located.
Many wireless communication systems communicate to a subscriber over a set of links from different base stations. The set of links is referred to as a primary channel. In many cases, however, the primary channel comprises only one link. The use of a set of links for a subscriber enables the system to select the best link for a particular instant of time and use that link to communicate with the subscriber. The best link is typically a communication link with the best quality of service (QoS). The QoS is typically a function of network variables that reflect the quality of a link. One example of such a network variable is the Frame Error Rate (FER). In CDMA systems and other wireless systems signals are transmitted as frames where each frame is a block of bits. When a frame is received with at least one error, it is said to be an error frame. The FER is the ratio of the number of error frames received to the total number of frame received during a defined period of time. Thus the communication system is able to communicate with a subscriber through a primary channel comprising at least one communication link. The communication system designates the linkxe2x80x94from the set of linksxe2x80x94which has the most favorable conditions (e.g., lowest FER)xe2x80x94for communication as the anchor leg. Accordingly, most of the time most of the communication between a subscriber and the system is done through the anchor leg.
Many communication systems now allow subscribers to have simultaneous access to more than one channel. A subscriber, while using a primary channel, may request a secondary channel. For example, a subscriber using the primary channel for low speed data may want to have access to a secondary channel to run additional applications at higher data rate throughput. Communication signals associated with the additional applications would then be exchanged over the secondary channel. In a multi-link primary channel, the anchor leg is usually selected for the secondary channel. However, prior to granting a subscriber access to the secondary channel, the communication system first assesses the subscriber""s resource requirements and then determines whether the required resources are available. The resources are the various communication assets such as the number of radios available from the selected link, the amount of power available per link, the bandwidth available per link and maximum information rate supportable at each link. The information rate and the amount of power supportable for transmission at a particular link are two critical resources whose usage system providers desire to apply in a most efficient manner.
Prior to providing a requesting subscriber access to a secondary channel, the communication system determines whether the secondary channel can support the information rate being requested by the subscriber. Some secondary channels support only one information rate, while other secondary channels support multiple data rates. If the secondary channel supports one information rate, the system determines whether there is sufficient power to transmit information at that rate. If the secondary channel supports multiple information rates, the system determines the best information rate based on the amount of power available and the RF conditions. Further, the communication system has to determine the amount of initial power it is to allocate to the secondary channel.
Most wireless communication systems perform some sort of power control technique so as to use their power efficiently. In many such techniques, the power made available to each primary channel is continually adjusted to compensate for changing system conditions. Communication system providers (i.e., owners and operators of communication system equipment) want to avoid circumstances where too much power is routed to one or a group of subscribers while other subscribers cannot gain access to the system because of insufficient power. The radio equipment at the base stations are thus controlled on a system wide basis to maintain a relatively efficient use of power throughout the system.
Typically the communication system allows the secondary channel to use a subset of the connections used by the primary channel. For example, the primary channel is in three way soft hand off with three base stations, while the communication system assigns a secondary channel to the same user from one of the links from the set of links that make up the primary channel. In general the communication system would assign a subset of the primary channel active connections to the secondary channel in order to reduce the network equipment resources needed to support the secondary channel connection. The most efficient method is to use only one connection for the secondary channel; usually the anchor leg.
In principle the transmitted power of each primary channel connection is the same, even if it changes in time and the path loss of each of the connections is different. Lack of knowledge of the path loss imbalance among the connections of the primary channel introduces errors in the estimate of the initial power required by the secondary connection; this is especially true when the secondary channel uses a subset of the primary channel connections. When only one link is being used for the secondary channel, error in the estimate of the initial power is critical because there is no diversity gain in the secondary channel. Diversity gain refers to a multi-link channel where information can be exchanged in any one of the links; that is, there are alternative link.s for exchanging information. A secondary channel having only one link has no alternative; it thus becomes critical that the one link operate satisfactorily in order to avoid relatively high FER.
An error in the estimate of the secondary channel initial power would introduce inefficiencies that impair the performance of the communication system. For example, if the system underestimates the amount of initial power it allocates to the secondary channel, the usage of the secondary channel may not last long enough for the power control technique being used to adjust to the correct amount of power for acceptable communications. In such a case, the initial power estimate is too low and the result is a relatively high FER. The system would then increase the power at some later time so as to reduce the FER, but at that point the communications over the secondary channel may have been terminated by the subscriber. Conversely, the system may have allocated too much initial power or a high value of the Signal to Ratio (SNR) setpoint to the secondary channel which results in the system not allowing access to other requesting subscribers because there is relatively little power remaining. Under these circumstances the power control technique being used should restore the correct secondary channel power level. However, when the duration of transmission of the secondary channel is short and the initial value of the power or the SNR setpoint is too high, the power control technique being used does not have the time to converge to the best values. In this case the service provider will use excessive power for the whole duration of the secondary channel transmission.
In these cases, the power control technique used by the system was not helpful because the initial power estimate for the secondary channel was not a sufficiently accurate estimate of the actual initial power requirement.
What is therefore needed is an algorithm that provides a relatively accurate estimate of the amount of initial power and SNR setpoint to be made available to a secondary channel to allow a communication system to perform power control techniques on the secondary channel enabling the communication system to use its power efficiently.
The present invention provides a method for determining a relatively accurate value for the initial power and initial SNR setpoint to be allocated to a secondary channel of a wireless communication system operating at a certain information rate. The method first calculates an initial SNR setpoint based on the SNR setpoint of an associated primary channel and system offset values. The calculated SNR setpoint, the information rate of the secondary channel and primary channel system parameters are then used to calculate the initial transmission power level for the secondary channel. The initial transmitted power of the secondary channel is calculated so that its value and variance are both within thresholds set by the service provider of the communication system.